Except it’s not Bernoulli’s principle, but rather the process of entrainment, where air in the region surrounding the air stream that he is blowing gets caught up in that air stream, multiplying the amount of air by a large factor. This air is directed into the bag due to the direction of the original air stream, thereby inflating the bag.

https://video.twimg.com/amplify_video/1784042706114584576/vid/avc1/718x1280/DQ5MgRs3i4tDHjz3.mp4?tag=14